1. Field of Invention
The present invention relates generally to semiconductor processing equipment. More particularly, the present invention relates to a CI-core or an EI-core actuator which enables travel to efficiently occur in a transverse direction.
2. Description of the Related Art
For precision instruments such as photolithography machines which are used in semiconductor processing, factors which affect the performance, e.g., accuracy, of the precision instrument generally must be dealt with and, insofar as possible, eliminated. When the performance of a precision instrument is adversely affected, as for example by excessive heat or weight, the integrity of a device formed using the precision instrument may be compromised.
Magnetic levitation stage assemblies are often used in photolithography processes to support a wafer that is being scanned. In magnetic levitation stage assemblies, magnetic actuators are used to allow a stage, e.g., a fine stage, to effectively undergo non-contact positioning. Such magnetic actuators produce a suspension or levitation force which allows a stage to be scanned or otherwise positioned without the stage having to substantially contact any surface.
The use of a magnetic levitation stage allows the weight associated with the stage to be substantially supported without requiring a relatively large amount of energy or current, and also allows the stage to be accurately positioned. An example of a magnetic levitation stage is described in co-pending U.S. patent application Ser. No. 10/272,930, filed Oct. 18, 2002, which is incorporated herein by reference in its entirety.
FIG. 1a is a diagrammatic representation of a CI-core actuator with a single coil which is capable of generating a force that may be used as a magnetic levitation force. A CI-core actuator 100 includes a C-core 102 and an I-core 104 which are separated by a gap 108. A coil 106 is substantially wound around a top-portion of C-core 102. If coil 106 is provided with current, coil 106 and the top-portion of C-core 102 form an electromagnet. When gap 108 is relatively small, as for example less than or equal to approximately one millimeter (mm) in size, CI-core actuator 100 may operate efficiently in that a relatively large amount of force may be generated for a relatively low amount of power. As will be understood by those skilled in the art, however, the need to maintain a relatively small gap 108 is not conducive to a relatively long travel in a transverse direction for a component such as a stage (not shown) that is to be moved using CI-core actuator 100.
Current may be provided through coil 106 such that a magnetic flux is generated with respect to C-core 102. The flux effectively penetrates gap 108 to create an attraction between C-core 102 and I-core 104. An attraction force, F, may be defined as follows:
      F    ≈                            N          2                ⁢                  i          2                ⁢        A        ⁢                                  ⁢                  μ          o                            4        ⁢                              (            gap            )                    2                      =      k    ⁢                  i        2                              (          gap          )                2            
where N is the number of turns of coil 106, i is the current flowing through coil 106, A is an area of one side of C-core 102, μo is the permeability of air, and (gap) represents the size of gap 108. The attraction force is generally more efficient for relatively a relatively small gap 108. When C-core 102 and I-core 104 are formed from a relatively high permeability iron material, the reluctance of the overall magnetic circuit is dominated by the reluctance across gap 108. However, the reluctance of the overall magnetic circuit is affected by the iron material in that the iron causes the overall magnetic path length to increase.
Instead of coil 106 being substantially wound around a top portion of C-core 102, a plurality of coils may instead be wound around “legs” of a C-core. The winding of coils around the legs of a C-core is less complicated to manufacture than the winding of a coil around a top portion of a C-core. FIG. 1b is a diagrammatic representation of a CI-core actuator with a plurality of coils. A CI-core actuator 110 includes a C-core 112 and an I-core 114 which are separated by a gap 118. Coils 116 are wound around sides of C-core 116 as shown.
While CI-core actuators are often used in magnetic levitation stage devices, EI-core actuators may be used as well. With reference to FIG. 1c, an EI-core actuator will be described. An EI-core actuator 120 generally includes an E-core 122 and an I-core 124 which are separated by a gap 128. A coil 126 may be wound around a “leg” of E-core 122, as shown, such that when current passes through coil 126, flux is substantially generated to cause an attraction between E-core 122 and I-core 124. As CI-core and EI-core actuators provide attractive forces, they are commonly arranged in opposing pairs.
CI-core or EI-core actuators may be used to facilitate movement of a stage, as for example a stage that is a magnetic levitation stage. Typically, at least a plurality of pairs of CI-core or EI-core actuators is used in a magnetic levitation stage apparatus. FIG. 2a is a diagrammatic side-view representation of a stage which carries a plurality of C-cores of a CI-core. A stage 204 includes a plurality of C-cores 208, and is arranged to move beneath a relatively large plate 212 which serves as an I-core that corresponds to C-cores 208. Such movement may be accomplished using linear motors. Since C-cores 208 are supporting weight at all times, relatively significant heat may be generated, and coolant is generally needed to provide cooling to stage 204. Further, when C-cores 208 move, substantially any wires or cables (not shown) coupled to C-cores 208 or to coils (not shown) wound around C-cores 208 must also move. Such wires and cables include power cables and cooling hoses. Wire and cables add disturbance forces to stage 204 which may adversely affect the movement and positioning performance of stage 204.
In addition, when C-cores 208 are supported on stage 204, the overall weight carried or otherwise supported by stage 204 increases. Increasing the weight carried or otherwise supported by stage 204 increases the amount of force needed to drive stage 204 and, hence, the power requirements of stage 204, which is often undesirable.
To reduce power requirements and complications associated with positioning C-cores 208 on stage 204 such that C-cores 208 effectively move with stage 204, C-cores may be substantially fixed to a non-moving portion of an overall stage device. With respect to FIG. 2b, a stage which carries an I-core portion of a CI-core actuator while the C-core portion of the CI-core actuator remains substantially fixed will be described. A stage 224 effectively includes an I-core surface (not shown), and is arranged to scan beneath C-cores 228 which are coupled to a substantially fixed surface 232. While the use of stage 224 which does not include moving coils associated with C-cores 228 substantially eliminates issues associated with the implementation of moving coils, stage 224 generally needs to be larger in a stroke direction than the maximum stroke in the stroke direction. That is, stage 224 is sized such that a dimension of stage 224 along a particular axis is larger than the maximum stroke along that axis. As a result, the size of a stroke which stage 224 may undergo is effectively limited by the size of the stage 224.
Further, the amount of force associated with each C-core 228 typically changes depending upon the location of a center of gravity 234 of stage 224, i.e., a force ratio associated with the force output of each C-core 228 changes. Typically, more force and, hence, more power is required in whichever C-core 228 is nearer to center of gravity 234. In addition, control dynamics generally change with the position of each C-core 228. When center of gravity 234 is located substantially at a mid-line between C-core 228a and C-core 228b, then forces generated using C-core 228a and C-core 228b may be substantially the same. However, when center of gravity 234 is located closer to C-core 228a, for example, then C-core 228a generally must generate more force than C-core 228b. Adjusting the amount of force associated with each C-core 228 depending upon where center of gravity 234 is located is often relatively complicated.
The lifting point associated with a stage also moves when the stage moves. This may cause changing distortion in the stage, which may adversely affect the performance of the stage. When the precision with which the stage may be positioned is affected, wafer fabrication processes which utilize the stage may be compromised.
For at least the reasons stated above, cooling issues and stroke issues associated with the use of CI-core actuators in stage assemblies as discussed with respect to FIGS. 2a and 2b generally do not efficiently allow for relatively long travel transverse to the force direction of the CI-core actuators. Although the use of voice coil motors in a stage apparatus such as a magnetic levitation stage apparatus may be effective in allowing for relatively longer travel in a transverse direction, voice coil motors generally operate less efficiently than CI-core and EI-core actuators. In addition, voice coil motors may either require the use of moving magnets, which add weight to a moving stage, or of moving coils, which gives rise to issues associated with moving cables and cooling.
Therefore, what is needed is a system and a method which enables a stage such as a magnetic levitation stage to have relatively long travel in a transverse direction without significant cooling concerns, stroke length concerns, or issues with moving cables. More specifically, what is desired is a CI-core actuator or an EI-core actuator which allows for long travel in a transverse direction.